Thermal spraying powders are used for producing coatings on substrates. Pulverulent particles are thereby introduced into a combustion or plasma flame directed onto the (mostly metallic) substrate which is to be coated. The particles melt in the flame, entirely or to some extent, and impact the substrate, where they solidify and, in the form of solidified “splats”, form the coating. Thermal spraying can produce coatings up to a layer thickness of a number of mm. A frequent application of thermal spraying powders is the production of antiwear layers. Thermal spraying powders typically involve a subgroup of cermet powders which firstly comprise a hard material, most frequently carbides, such as tungsten carbides, chromium carbides, and molybdenum carbides, and secondly comprise a matrix composed of metals, for example, cobalt, nickel, and alloys of these with chromium, or else less frequently comprise iron-containing alloys. Thermal spraying powders and spray layers produced therefrom are therefore composite materials.
Coatings, like bulk materials, have empirically determinable properties. Among these are hardness (for example, Vickers, Brinell, Rockwell and Knoop hardness), wear resistance (for example, ASTM G65), cavitation resistance, and also corrosion performance in various media. Corrosion resistance is increasingly important during selection of spraying materials since many antiwear layers must exhibit dependable stability under acidic conditions in chemically aggressive environments (examples being use in the oil and gas industry, paper industry, chemicals industry, the food-and-drink industry, and the pharmaceutical industry, often with the exclusion of oxygen). This applies by way of example to displaceable parts of valves and to piston rods when acidic mineral oil or natural gas are conveyed in the presence of chlorides or seawater. There are also many applications in the food-and-drink industry, and also the chemicals industry, where wear and corrosion exert negative synergy and thus reduce the lifetime of antiwear coatings.
The corrosion of spray layers in acidic liquids and in the presence of chlorides takes place in accordance with the principle known to apply to cemented hard materials: the matrix alloy is attacked, and ions of the matrix metals are thus liberated. This provides access to the hard materials of the spray layer, and ablation of the spray layer takes place. When tribological wear is superposed, there is then a negative synergy from wear and corrosion. Corrosion performance is further reduced by the fact that contact corrosion can occur between the hard materials and the matrix, the matrix therefore being more susceptible to corrosion in the composite material than it would be alone. This is equally observed in cemented hard materials.
Various materials have become established as thermal spraying powders for producing spray layers for the abovementioned applications, examples including WC—CoCr 86/10/4 or WC—CoNiCr 86/9/1/4, WC—Cr3C2-Ni and Cr3C2-NiCr. A feature shared by all of the abovementioned is that they comprise Cr in the matrix since this ensures corrosion-resistance.
Another material is WC—NiMoCrFeCo 85/15 which is obtainable commercially in the form of thermal spraying powder (Amperit® 529 from H. C. Starck GmbH, D). Its matrix is composed of an alloy similar to Hastelloy® C. Although Hastelloy® C is used successfully in acidic media, this alloy lacks wear resistance. However, it exhibits poorer properties as a matrix alloy in a composite “spraying powder” or “spray layer” material.
Analogous considerations apply to the chromium carbide-NiCr (80/20) materials available on the market. The good acid resistance of NiCr 80/20 cannot be transferred to the thermal spraying powder with chromium carbides or to the spray layer produced therefrom.
Fe-based matrix alloys, for example, those derived from austenitic stainless steels such as 316L, or based on FeCrAl 70/20/10 as described in DE 10 2006 045 481 B3, fail in an acidic environment at low pH.
When any of the abovementioned materials in the form of compacted spray powder is exposed to hydrochloric acid, sulfuric acid, or citric acid, it exhibits weakness in at least one of these media, or a weaknesses in mechanical properties.